Surface acoustic wave (SAW) delay lines are employed for a number of signal processing applications. Frequency synthesizers, in particular, are one type of device which requires a tap delay line that can provide a wide range of delays, some of which may be relatively long. A SAW tap delay line may be used to advantage in such a device.
Prior wideband tapped SAW delay lines were constructed by using an input transducer which is in line with a number of separate output transducer taps along a single acoustic path. The output taps were lightly coupled in order to reduce losses due to reflections. Even with light coupling, reflections are a serious problem with such a device, particularly if there are a relatively large number of taps because the reflection of each tap may be directed straight into its neighboring taps.
Another disadvantage or limitation of such a prior type of SAW tap delay line is that the maximum delay path is limited by the length of the substrate. Since the piezoelectric materials generally employed for SAW devices may currently be manufactured only on relatively small wafers, the maximum delay can be substantially limited. For example, a maximally large size wafer of lithiumniopate (LiNbO.sub.2) is three inches in diameter. With this size wafer, a maximum delay on the order of 18 microseconds is provided.
In order to get around this length limitation, there have been folded path geometries which used multiple-strip, coupled track changers, so that the acoustic waves are reflected from one path into another path after the first path has been substantially traversed. The performance of this type of device, however, is substantially limited by excessive loss due to spurious reflections.
The SAW delay line with multiple reflective taps of the present invention provides an improved structure with longer delays and lower losses.